Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence.
More pertinent to the present inventions described herein, Deep Brain Stimulation (DBS) has been applied therapeutically for well over a decade for the treatment of neurological disorders. DBS and other related procedures involving implantation of electrical stimulation leads within the brain of a patient are increasingly used to treat disorders, such as Parkinson's disease, essential tremor, seizure disorders, obesity, depression, obsessive-compulsive disorder, Tourette's syndrome dystonia, and other debilitating diseases via electrical stimulation of one or more target sites, including the ventrolateral thalamus, internal segment of globus pallidus, substantia nigra pars reticulate, subthalamic nucleus (STN), or external segment of globus pallidus.
DBS has become a prominent treatment option for many disorders, because it is a safe, reversible alternative to lesioning. For example, DBS is the most frequently performed surgical disorder for the treatment of advanced Parkinson's Disease. There have been approximately 30,000 patients world-wide that have undergone DBS surgery. Consequently, there is a large population of patients who will benefit from advances in DBS treatment options. Further details discussing the treatment of diseases using DBS are disclosed in U.S. Pat. Nos. 6,845,267 and 6,950,707, which are expressly incorporated herein by reference.
Implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of a pulsed electrical waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, duration, and frequency of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).
In the context of DBS, a multitude of brain regions may need to be electrically stimulated in order to treat one or more ailments associated with these brain regions. To this end, multiple stimulation leads are typically implanted adjacent the multiple brain regions. In particular, multiple burr holes are cut through the patient's cranium as not to damage the brain tissue below, a large stereotactic targeting apparatus is mounted to the patient's cranium, and a cannula is scrupulously positioned through each burr hole one at a time towards each target site in the brain. Microelectrode recordings may typically be made to determine if each trajectory passes through the desired part of the brain, and if so, the stimulation leads are then introduced through the cannula, through the burr holes, and along the trajectories into the parenchyma of the brain, such that the electrodes located on the lead are strategically placed at the target sites in the brain of the patient.
Stimulation of multiple brain structures (i.e., different functional regions of the brain) with different sets of stimulation parameters has been shown to be useful. For example, stimulation of the Pedunculopontine (PPN) and Subthalamic Nuclei (STN) at different frequencies has been shown to be beneficial (see Alessandro Stefani, et al. “Bilateral Deep Brain Stimulation of the Pedunculopontine and Subthalamic Nuclei in Severe Parkinson's Disease,” Brain (2007); 130 1596-1607). In another DBS example, one frequency is used to optimize treatment of tremor and rigidity, while another frequency is used to treat bradykinesia (see U.S. Pat. No. 7,353,064).
Thus, if the same set of stimulation parameters is used to stimulate the different brain structures, either (1) one brain structure may receive optimal therapy and the other brain structure may receive poor therapy, or, (2) both brain structures may receive mediocre therapy. Thus, to maximize the therapeutic effects of DBS, each brain structure may require different sets of stimulation parameters (i.e. different amplitudes, different durations, and/or frequencies).
One way that prior art DBS techniques attempt to stimulate several brain structures using different stimulation parameters is to implant multiple leads adjacent the different regions of the brain, and to quickly cycle the stimulation through the brain structures with the different stimulation parameters. In some applications, such as the treatment of chronic pain, this effect may be unnoticeable; however, the brain is a complex system of rapidly transmitting electric signals, and the effect of rapid cycling may produce a “helicopter effect” that may undesirably result in ineffective treatment and/or side-effects such as seizures.
Another way that prior art DBS techniques attempt to stimulate several brain structures using different stimulation parameters is to connect the multiple leads to multiple neurostimulators respectively programmed with different stimulation parameters. However, this increases the cost of the procedure, increases the length of the procedure, and increases the risks associated with the surgery.
Another approach is to use multiple timing channels when applying electrical stimulation to different brain structures. Each timing channel identifies the combination of electrodes used to deliver electrical pulses to the targeted tissue, as well as the characteristics of the current (pulse amplitude, pulse duration, pulse frequency, etc.) flowing through the electrodes. Because prior art neurostimulation systems are incapable of simultaneously controlling the generation of electrical pulses (e.g., either because they comprise only one anodic electrical source and one cathodic electrical source or otherwise because one or more stimulation parameters used to define one electrical pulse may be overwritten with one or more stimulation parameters used to define a subsequent overlapping electrical pulse), the use of multiple timing channels can often lead to issues due to the potential of an overlap in electrical pulses between two or more timing channels. These neurostimulation systems may time-multiplex the pulsed electrical waveforms generated in each of the multiple channels to prevent electrical pulses in the respective channels from overlapping each other.
For example, with reference to FIG. 1, one prior art neurostimulation controller 1 that is capable of controlling output stimulation circuitry 2 to output up to four pulsed electrical waveforms respectively over four timing channels in accordance with four stimulation parameter sets. The output stimulation circuitry 2 includes a single anodic current source 3a and an associated decoder 4a (or bank of decoders), and a single cathodic current source 3b and an associated decoder 4b (or bank of decoders). The decoder 4a is configured for decoding a digital code defining an anodic electrode combination (i.e., the active anodic electrodes) and amplitude values for the anodic electrode combination, and the decoder 4b is configured for decoding a digital code defining a cathodic electrode combination (i.e., the active cathodic electrodes) and amplitude values for the cathodic electrode combination.
The neurostimulation controller 1 comprises a number of registers 5 (in this case, four registers 1-4), each of which digitally stores certain parameters of one of the four stimulation parameter sets, and in particular, the electrode combination (i.e., the active electrodes) and amplitude and polarity (cathode or anode) of each of the active ones of the electrode combination. The neurostimulation controller 1 further comprises a number of timers 6 (in this case, four timers 1-4), each of which controls the pulse duration and frequency of one of the four stimulation parameter sets by outputting a high/low signal.
The neurostimulation controller 1 further comprises a multiplexor/selector 7 that outputs the digital contents (electrode combination, amplitude, and polarity) of a selected one of the registers 5 to the decoders 3 of the stimulation output circuitry 1 when the signal output by the respective timer 6 to the multiplexor/selector 7 is high (i.e., a logical 1 on one of the timers 6 gates the associated register 5 to the output of the multiplexor/selector 7). The stimulation output circuitry 2 then outputs an anodic electrical pulse and a cathodic electrical pulse in accordance with the electrode combination, amplitude, and polarity defined by the digital contents of the respective register 5 and decoded by the decoders 3, and the pulse width and frequency defined by the respective timer 6.
The neurostimulation controller 1 further comprises an arbitrator 8 for serially selecting the timing channels in which anodic and cathodic pulses will be output by the stimulation output circuitry 2 by serially turning on the timers 6, and thus, serially outputting the digital contents of the respective register 5 to the stimulation output circuitry 5. The arbitrator 8 selects the timing channels in a manner that prevents overlap of electrical pulses between the channels to avoid the aforementioned problems associated with attempting to generate overlapping pulses using single-source output circuitry. Notably, for the specific architecture illustrated in FIG. 1, preventing overlap of electrical pulses will ensure that information of a current electrical pulse (i.e., the digital contents obtained from one of the registers 5) stored within the decoders 3 of the stimulation output circuitry 2 is not overwritten with information of an overlapping electrical pulse (i.e., the digital contents obtained from another of the registers 5) when subsequently stored in the decoders 3 of the stimulation output circuitry 2.
If the frequencies of two pulsed electrical waveforms are the same or a harmonic of the other, the electrical pulses can be easily spaced in time within the respective channels, such that they do not coincide, as illustrated by the pulsed electrical waveforms in FIG. 2. For purposes of simplicity, only the anodic portion of the pulsed electrical waveforms is shown. When the frequencies of two pulsed electrical waveforms are not the same or otherwise not a harmonic of each other, the pulses of the pulsed electrical waveform with the faster frequency will “walk” over the pulses in the other pulsed electrical waveform, and therefore, there will be occasions when the pulses in the respective channels will need to be simultaneously generated, as illustrated in FIG. 3.
However, when there is only one source for each polarity, as shown in FIG. 1, or at least, when there are one or more non-dedicated sources (i.e., a source that can be shared by multiple electrodes), two electrical pulses of the same polarity cannot be generated simultaneously due to the potential of digitally overwriting the electrode combination information of the first electrical pulse with the electrode combination information associated with the second electrical pulse. Thus, even though multiple pulsed electrical waveforms can be generated in multiple channels, they must all have related frequencies to maintain a constant period, unless at least one pulsed electrical waveform is modified.
For example, in one embodiment, the arbitrator 8 uses a method known as the “token” method to prevent overlap of stimulation pulses between channels by modifying one or more of the pulsed electrical waveforms. This method allows an electrical pulse to be transmitted in the timing channel with the “token,” while the other timing channels wait their turn. Then, the “token” is passed to the next timing channel. However, if the channels overlap, such that they need the “token” at the same time, transmission of an electrical pulse within the second channel must wait until the end of the transmission of the electrical pulse in the first timing channel. The arbitrator 8 accomplishes this by putting the timer 6 associated with the subsequent electrical pulse on hold while the output of the multiplexor/selector 7 is in use.
The “token” method may best be understood with reference to FIG. 4. As there shown, a first pulsed electrical waveform 9a having a first frequency is transmitted within timing channel A, and a second pulsed electrical waveform 9b having a second frequency is desired to be transmitted within timing channel B. Because timing channel A has the “token,” the pulses of the second pulsed electrical waveform 9b that are to be transmitted in timing channel B must be “bumped” each time they overlap with the pulses of the first pulsed electrical waveform 9a. As can be seen in the bumped pulsed electrical waveform 9c, when a pulse is bumped (shown by the horizontal arrows), the next pulse relies on the new (bumped) pulse for timing. Thus, the next pulse is “double bumped”: once when the previous pulse is bumped and a second time when it overlaps a pulse of the pulsed electrical waveform 9a transmitted in the timing channel A. As a result, the frequency of the pulses in the second pulsed electrical waveform 9b is forced (i.e., locked) into the frequency for the first pulsed electrical waveform 9a, resulting in a pulsed electrical waveform 9d that has a frequency twice as small as the desired frequency.
One adverse result of using the token method is that the frequency of the electrical pulses transmitted in the second timing channel gets “locked” to (i.e. matches) the frequency of the electrical pulses transmitted in the first timing channel; alternatively, one can get galloping or clumping of electrical pulses. Therefore, when the occurrence of electrical pulses is pushed out in time, stimulation therapy may become ineffective or even harmful for tissue regions, such as brain structures to be stimulated in DBS applications, that require stimulation at specific, regular frequencies (See Birno M J, Cooper S E, Rezai A R, Grill W M, Pulse-to-Pulse Changes in the Frequency of Deep Brain Stimulation Affect Tremor and Modeled Neuronal Activity, J. Neurophysiology, 2007 September; 98(3): 1675-84.
There, thus, remains a need to provide an improved technique for independently operating multiple stimulation channels in a neurostimulation system where at least one electrical source in the neurostimulation system is shared by a plurality of electrodes.